When do U use 'in time','on time','at time'?

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Answer

in time 是 及时 的意思，on time 是 按时 的意思， at time 是 在某个时间 的意思。

Answer

On time: punctual, not late. 
If something happens ON time, it happens at the time which was planned: 
'The 11.45 train left on time.' (= it left at 11.45) 
'I'll meet you at 7.30' 'OK, but please be ON time.' 
The opposite of ON time is 'late': 
'Be on time. Don't be late.' 

In time (for something / to do something) = soon enough 
'I'm in a hurry. I want to be home IN time to see the game on television.' 
= soon enough to see the game. 
The opposite of IN time is 'too late' 

 at the same time / at the time / at the appoinyed time / at the usual time / at that time ...